In the semiconductor industry it is common practice to produce high purity silicon by a process known as chemical vapor deposition (“CVD”). In brief, certain substances having silicon content are heated to high temperatures within a reaction chamber causing them to undergo decomposition, while in the vapor state, and produce elemental silicon. Depending on the design of the reaction chamber, and whether or not it additionally contains deposition surfaces, the elemental silicon may be collected as a powder or as a rod. Such silicon is frequently referred to as polysilicon or polycrystalline silicon.
One of the widely practiced conventional methods of polysilicon production is via chemical vapor deposition of polysilicon in a thermal decomposition reactor, and is generally identified as the Siemens method. In this method polysilicon is deposited by decomposition of a silicon-containing gas such as for example trichlorosilane or monosilane (SiH4), within the thermal decomposition reactor onto high-purity, joule or resistance heated, thin silicon filaments. Silicon deposits on the filaments, thereby growing elongated polysilicon bodies of increasing diameter, while the polysilicon bodies are maintained at elevated temperatures, typically from 700° C. to 1200° C.
Stresses are stored in the elongated polysilicon bodies following growth in a thermal decomposition reactor due to temperature differences across the diameter of the elongated bodies and/or throughout the length of the elongated bodies. Once growth is complete, and the rod begins to cool, the variance in temperature during growth manifests as stress due to the coefficient of thermal expansion. The magnitude of the stored stress increases with diameter. The filaments, and resulting elongated bodies formed in a thermal decomposition reactor, typically have an inverted U-shaped configuration with two vertical portions and a relatively horizontal bridge portion between the tops of the two vertical portions. The two vertical portions of the elongated body are connected to the bridge at bend, or corner, portions. As an elongated polysilicon body shrinks due to thermal contractions, the bridge portion tends to separate from the vertical portions of the elongated body. A fracture can propagate down the vertical portion of the elongated body, e.g., for a distance of 200-1000 mm. Intact vertical portions, or rods, obtained from an elongated polysilicon body are of greatest commercial value due to their length with uniform diameter. Fractures reduce the yield of such vertical portions. Fractured rods may not meet customers' minimum length requirements. In some cases, product losses reach 50% due to fractures. Accordingly, there is a need to mitigate or control stresses in the elongated polysilicon body, thereby mitigating or controlling fractures produced by the stresses.